These instructions assume that you have some object modeled in Blender upon which you want to apply this Leather shader in Cycles. From the File menu, choose Append. Navigate to the .blend file you downloaded containing the Leather shader. Within the NodeTree folder, select Leather. Now, you will have a new Shader in Cycles, like Diffuse or Glossy, which you can apply to your object. When you add nodes to the Cycles material for your object, you will find the Leather shader under "Group" in the popup menu. Hope this helps!

Just... WOW! I opened the node editor to check, and saw the group and was like "let's see what's in this group, shouldn't be too much"... a few seconds later I was laughing because I had no idea what I saw anymore xD really good job!